1. Field of the Invention
The present invention relates generally to the field of optical devices for imparting or using phase delays and gratings, and more particularly, to Mach-Zehnder interferometer devices with fiber gratings.
2. Technical Background
Fiber optic Wavelength Division Multiplexed (WDM) networks carry information in multiple wavelength channels. WDM networks need mechanisms for adding and dropping channels. This is done through Wavelength Add/Drop Multiplexers commonly known as WADM""s. It is highly desirable in WDM networks to have the capability to partially alter the traffic at each repeater by adding and/or dropping one or several service channels while leaving the remainder of channels to propagate unchanged. There is considerable interest in using optical WADM""s to implement add/drop functionality for reconfigurable and transparent networks.
One approach to WADM architecture uses multiplexers or demultiplexers, known as MUX or DEMUX devices, to add or drop the desired channels. MUX/DEMUX devices, however, often have relatively large insertion losses and high front end costs. These are drawbacks to their use in many applications.
Another approach to implement WADM architectures uses fiber Bragg gratings to select the add/drop channel. In general, fiber Bragg gratings reflect light of a wavelength equal to 2n xcex9 (where n is the index of refraction of the waveguide and xcex9 is the grating period), while other wavelengths propagate through unchanged. One approach to a WADM architecture based on fiber Bragg gratings includes a Mach-Zehnder interferometer with a fiber Bragg grating in each arm. In this approach, grating position and period must be closely matched to tight tolerances to achieve the interferometric effect that leads to the add/drop function. This results in high manufacturing costs.
What is needed is an optical device that makes possible a WADM architecture that is relatively low cost, and that can be implemented in a device that is relatively easy to manufacture.
One aspect of the present invention is a Mach-Zehnder device with first and second waveguiding arms. The input ends of the first and second arms are coupled, and likewise the output ends are coupled. A delay element in the first arm delays propagation of light at a first wavelength relative to propagation of light at a second wavelength. In another aspect, the invention includes a delay element made up of a grating having a period which causes coupling of the first wavelength from a first mode into a second mode.
In another aspect, the invention includes an apparatus that can impart a phase delay to a portion of an optical signal of a predetermined wavelength. This embodiment includes an optical waveguide that can propagate light at that wavelength in first and second propagation modes. A grating in the waveguide has a period that causes the portion of the light at that wavelength to couple from the first into the second propagation mode. In another aspect, the invention includes a second grating in the waveguide positioned downstream from the first grating and having a period which causes coupling of the portion of light from the second propagation mode back into the first propagation mode.
In another aspect, the invention includes an optical waveguide in which light at a wavelength xcex can propagate in at least a first and a second mode. The waveguide has an effective index of refraction n1 with respect to the first propagation mode of xcex, and an effective index of refraction n2 with respect to the second propagation mode. A grating formed in said waveguide and having a period approximately equal to(xcex/2)(n1xe2x88x92n2).
In another aspect, the invention includes a grating having a period that is a function of a variable physical parameter of the grating. This parameter may be, for example, temperature or strain.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.